A lithium-based composite oxide crystal with a nasicon-type crystal structure shows promise as a solid electrolyte material of a lithium ion battery due to its chemical stability and high lithium ion conductivity at room temperature. To achieve high battery performance, impurity reduction, high crystallization, and microparticulation are required for the solid electrolyte material.
In this regard, a lithium-based composite oxide crystal represented by Formula (I)Li1+xAlxTi2−x(PO4)3(0≤x≤1.0)  (I)(hereinafter, the above crystal is called the “LTP or LATP crystal”, where, when x=0 in the above Formula (I), the crystal is an LTP crystal, and when 0<x≤1.0 in the above Formula (I), the crystal is a LATP crystal) has chemical stability and high lithium ion conductivity, which are required for the solid electrolyte material, and is free of a rare element and relatively easy to produce. Accordingly, the LTP or LATP crystal, among other lithium-based composite oxide crystals, holds great promise as the aforementioned solid electrolyte material. Thus, impurity reduction, high crystallization, and microparticulation are strongly desired especially for the LTP or LATP crystal.
At present, the solid-phase method, sol-gel method, and vitrification method are generally used as a method of producing the LTP or LATP crystal, and any of these methods involves milling for microparticulation. Milling might pose problems, such as impurity contamination and strain occurring in the crystal structure, which might be a cause of a decrease in lithium ion conductivity. Furthermore, since a high-level milling technique is required for sharpening particle size distribution, a microparticulation technique which does not involve milling is sought for.
Therefore, there has been proposed a method of producing a porous body of LTP or LATP crystals, the method including vitrification of a plurality of oxides, as materials of the LTP or LATP crystals, together with Ca3(PO4)2, followed by thermal treatment and acid treatment of glass resulting from the vitrification (refer to Patent Literature 1).